They are used primarily to treat disorders where abnormal or excessive histamine release by inflammatory cells is thought to underly illness.This includes conditions such as: Other conditions such as hyper-reactive (vasomotor) rhinitis and pruritus of any cause are commonly treated with antihistamines, although there is little evidence that histamine plays a contributory role.Other common adverse effects in first-generation H-antihistamines include dizziness, tinnitus, blurred vision, euphoria, uncoordination, anxiety, increased appetite leading to weight gain, insomnia, tremor, nausea and vomiting, constipation, diarrhea, dry mouth, and dry cough.Infrequent adverse effects include urinary retention, palpitations, hypotension, headache, hallucination, and psychosis.There is little evidence that antihistamines used symptomatically to treat nonspecific itching have any effect greater than placebo.
The reason for their peripheral selectivity is that most of these compounds are zwitterionic at physiological p H (around p H 7.4).
First-generation antihistamines include diphenhydramine (Benadryl), carbinoxamine (Clistin), clemastine (Tavist), chlorpheniramine (Chlor-Trimeton), and brompheniramine (Dimetane).
Notwithstanding it is important to note that a 1955 study of "antihistaminic drugs for colds," carried out by the U. Army Medical Corps, reported that "there was no significant difference in the proportion of cures reported by patients receiving oral antihistaminic drugs and those receiving oral placebos.
H-antihistamines can be administered topically (through the skin, nose, or eyes) or systemically, based on the nature of the allergic condition.
The authors of the American College of Chest Physicians Updates on Cough Guidelines (2006) recommend that, for cough associated with the common cold, first-generation antihistamine-decongestants are more effective than newer, non-sedating antihistamines.